1. Field of the Invention
The present invention relates to an actuator for converting rotary motion of a feed screw into rectilinear motion by using a feed nut to allow a displacement member to perform rectilinear motion.
2. Description of the Related Art
An actuator, which is based on the use of a feed screw, has been hitherto used as a driving source for transporting a workpiece or the like. For example, as shown in FIG. 8, such an actuator 10 comprises a ball screw 16 with its one end which is connected to a rotary shaft 14 of a motor 12. The other end of the ball screw 16 is rotatably supported by a shaft support member 18. A feed nut 20 meshes with the ball screw 16. The feed nut 20 is surrounded by a displacement member 22. The displacement member 22 is formed with an attachment section 24 which protrudes at an upper portion of the displacement member 22 and which extends along its displacement direction. The attachment section 24 protrudes upwardly through a slit 28 which is formed at an upper portion of a frame 26 of the actuator 10.
When the motor 12 is operated, the rotary motion of the ball screw 16 is converted into rectilinear motion by the aid of the feed nut 20. The rectilinear motion is transmitted to the displacement member 22. Thus, the displacement member 22 makes displacement along the longitudinal direction of the actuator 10.
However, if the conventional actuator 10 as described above involves any axial deviation between the frame 26 and the ball screw 16, then the sliding resistance with respect to the inner wall of the frame 26 is increased when the displacement member 22 makes displacement, and the displacement action of the displacement member 22 is obstructed when the displacement member 22 makes displacement in the frame 26.
For this reason, when the ball screw 16 is assembled to the frame 26, then it is necessary that the both ends of the ball screw 16 are subjected to centering adjustment with respect to the rotary shaft 14 and the shaft support member 18, and it is also necessary that the ball screw 16 is subjected to centering adjustment with respect to the feed nut 20. Therefore, a problem arises in that the assembling operation is complicated.
Further, if the central portion of the ball screw 16 is warped, for example, when the actuator 10 has a lengthy size, or when a load of a workpiece or the like is exerted on the displacement member 22, then the sliding resistance between the displacement member 22 and the frame 26 is increased in the same manner as described above, and the displacement member 22 occasionally fails to make displacement. For this reason, it has been impossible to allow the actuator 10 to have a fairly long size, and it has been also necessary to restrict the load of the workpiece.